Luminescent or backlit displays such as LCD screens are widely used in various devices such as computers, mobile phones and television sets. Liquid Crystal Displays (LCD) are multi-layered systems comprising: a color filter layer, a liquid crystal layer and a backlight unit. The backlight unit is configured to produce a primary light which is guided towards the liquid crystal layer, while said liquid crystal layer is configured to modulate the transmission of light towards the color filters layer. Conventional color filters generally comprise an array of color filters, while each color filter form a sub-pixel and allow transmitting a defined range of wavelengths of the light and absorbing the other wavelengths of the light. A combination of color filters of different wavelength ranges generally forms a pixel from which a polychromatic light can be obtained. When colored lights are obtained from an array of pixels, an image can be viewed by the viewer.
In LCD displayer for example, the backlight unit comprises light sources configured to emit primary light and a polarizer configured to polarize said primary light. The backlight unit is configured to provide said polarized light to the liquid crystal layer, the color filter layer and the second polarizer. As said polarized light pass through the liquid crystal layer and the color filter layer, only the selected protion of the primary light will be transmitted through the second polarizer, such that an image can be viewed by the viewer.
The backlight unit is provided with multiple light sources. With the development of technologies, light-emitting diodes (LEDs) have started to be used for the backlight unit, which replace cold cathode fluorescent lamps (CCFLs). The LED has many advantages, as compared with the CCFL, in view of less power consumption, an extended lifespan, and a facilitated fabrication in a small size.
Furthermore, the color filter should be illuminated by a light with narrow luminescence spectra to increase the color purity and decrease the loss of energy. This will result in highly saturated shades with vivid, intense colors, while less saturated shades appear rather bland and gray.
We know from the prior art an illumination source comprising a light source coupled to a layer of phosphors. However, those phosphors have a rather large full width half maximum, typically larger than 70 nm. This results in poor color purity, leading to non-saturated colors and energy loss in the final display and lighting devices.
We also know an illumination source comprising a light source coupled to a color conversion layer, wherein the conversion layer comprises quantum dots. Indeed, quantum dots are currently used in display devices like phosphors. Quantum dots have a narrow fluorescence spectrum, approximately 30 nm full width at half maximum, and offer the possibility to emit in the entire visible spectrum as well as in the infrared with a single excitation source in the ultraviolet.
However, there is a real need for materials having a high stability for long term use when deposited on diodes, or LED, i.e. having a high stability in time and in temperature under a high photon flux. Indeed, when used on LED, nanoparticles must resist to temperatures higher than 100° C. and constant high-intensity illumination.
To ensure a high longterm stability, further chemical reaction between the surface of nanoparticles and environmental deteriorating species, such as water, oxygen or other harmful compounds, must be prevented during their use. However, the ligands commonly used to functionalize the surface of quantum dots do not protect efficiently said surface against reactions with deteriorating species or harmful compounds and thus do not enable the long-term performance required for display or lighting devices.
It is therefore an object of the present invention to provide an illumination source comprising a light source coupled to a color conversion layer comprising composite particles. Said composite particles comprise a plurality of nanoparticles, especially fluorescent nanoparticles, encapsulated in an inorganic material. The inorganic material forms a protective shell: i) to prevent degradation due to deteriorating species, harmful compounds or high temperature; ii) to drain away the heat and the electrical charges originating from the inorganic nanoparticles and the LED. Furthermore, composite particles can act as scatterers so that resulting light can be emitted in all directions. This illumination source will provide an intense resulting light comprising narrow fluorescence spectra as an alternative to the use of quantum dots.